Protein Kinases
The protein kinases comprise an exceptionally large family of eukaryotic proteins which mediate the responses of cells to external stimuli and are related by amino acid sequence homology within the so-called "catalytic domain" of the enzymes. To date, in excess of 100 unique members of the protein kinase family from a wide variety of eukaryotic organisms have been described and characterized at the amino acid sequence level. See, e.g., Hanks, et al. (Science, 241:42-52, 1988) which presents a sequence alignment of 65 protein kinase catalytic domains which range in size from about 250 to 300 amino acids and Hanks, et al. (Methods in Enzymol., 200:38-62, 1991) presenting a catalytic domain sequence alignment for 117 distinct protein kinase family members including a variety of vertebrate, invertebrate, higher plant and yeast species enzymes. The location of the catalytic domain within a protein kinase is not fixed. In most single subunit enzymes, the domain is near the carboxy terminus of the polypeptide while in multimeric protein kinases the catalytic domain takes up almost the entirety of the subunit polypeptide.
Protein kinases are generally classified into a protein-serine/threonine subfamily or a protein-tyrosine subfamily on the basis of phosphorylation substrate specificity. Among the many classes of enzymes within the protein-serine/threonine kinase subfamily are two distinct classes which have been designated casein kinase I and casein kinase II based on the order of their elution from DEAE-cellulose. The casein kinases are distinguished from other protein kinases by their ability to phosphorylate serine or threonine residues within acidic recognition sequences such as found in casein. Tuazon, et al., (Adv. in Second Messenger and Phosphoprotein Res., 23:123-164, 1991) presents a review of over 200 publications related to casein Iinase I and II, addressing the physicochemical characterization, recognition sequences, substrate specificity and effects on metabolic regulation for these two classes of enzymes. Casein kinase II is active as a heterotetramer and the complete amino acid sequences of human, rat, Drosophila and yeast species catalytic regions have been determined. Despite the fact that partially purified casein kinase I preparations have been obtained from cell nuclei, cytoplasm, and cell membranes of various plant and animal species, prior to the present invention, nothing was known concerning the primary structure of its enzymatically active monomeric subunit.
As of the time of the present invention, therefore, there existed a significant need in the art for information concerning the primary structure (amino acid sequence) of protein-serine/threonine kinase enzymes of the casein kinase I class. Such information, provided in the form of DNA sequences encoding one or more of these kinases (from which primary structures could be deduced), would allow for the large scale production of kinases by recombinant techniques as well as for determination of the distribution and function of these enzymes, the structural distinctions between membrane-bound and non-membranous forms, the potential ligand-receptor interactions in which these kinases interact, and the identification of agents capable of modulating ligand-receptor binding, kinase, and other activities.
DNA Recombination And Repair
Chromosomes experience single-stranded or double-stranded breaks as a result of energy-rich radiation, chemical agents, as well as spontaneous breaks occurring during replication among others. Although genes present in the chromosomes undergo continuous damage, repair, exchange, transposition, and splicing, certain enzymes protect or restore the specific base sequences of the chromosome.
The repair of DNA damage is a complex process that involves the coordination of a large number of gene products. This complexity is in part dependent upon both the form of DNA damage and cell cycle progression. For example, in response to ultraviolet (UV) irradiation, cells can employ photoreactivation or excision repair functions to correct genetic lesions. The repair of strand breaks, such as those created by X-rays, can proceed through recombinational mechanisms. For many forms of DNA damage, the cell is induced to arrest in the G2 phase of the cell cycle. During this G2 arrest, lesions are repaired to ensure chromosomal integrity prior to mitotic segregation.
Since the transfer of genetic information from generation to generation is dependent on the integrity of DNA, it is important to identify those gene products which affect or regulate genetic recombination and repair. Through the use of organisms with specific genetic mutations, the normal functional gene can be obtained, molecularly cloned, and the gene products studied.
In eukaryotes such as Saccharomyces cerevisiae, genetic studies have defined repair-deficient mutants which have allowed the identification of more than 30 radiation-sensitive (RAD) mutants (Haynes, et al., in Molecular Biology of the Yeast Saccharomyces, pp. 371, 1981; J. Game in Yeast Genetics: Fundamental and Applied Aspects, pp. 109, 1983). These mutants can be grouped into three classes depending upon their sensitivities. These classes broadly define excision-repair, error-prone repair, and recombinational-repair functions. The molecular characterization of yeast RAD genes has increased the understanding of the enzymatic machinery involved in excision repair, as well as the arrest of cell division by DNA damage.
The understanding of RAD genes and their expression products has become increasingly important as research continues to develop more effective therapeutic compositions. Often these new compositions appear quite effective against a particular disease condition, such as certain tumors, but prove to be too toxic for in vivo therapy in an animal having the disease. Indeed, these compositions can actually increase the likelihood of mutagenesis.
Most agents that are mutagenic or carcinogenic are in themselves unreactive, but are broken down to reactive intermediates in vivo. It is these reactive intermediates which interact with DNA to produce a mutation. This event is thought to be the initial step in chemical carcinogenesis. Mutations in a large number of genes affect the cellular response to agents that damage DNA. In all likelihood, many of these mutated genes encode enzymes that participate in DNA repair systems. Consequently, when the repair system is compromised, the cells become extremely sensitive to toxic agents. Although the DNA may revert to normal when DNA repair mechanisms operate successfully, the failure of such mechanisms can result in a transformed tumor cell which continues to proliferate.
Although there are currently available tests to determine the toxicity or mutagenicity of chemical agents and compositions, there are limitations in both laboratory screening procedures and animal toxicity tests. These limitations include extrapolating laboratory data from animals to humans. There is often a large measure of uncertainty when attempting to correlate the results obtained in laboratory animals with effects in human subjects. In most cases, doses of the test drug have been used in the animal which are too high to be safely administered to humans. In addition, some types of toxicity can be detected if the drug is administered in a particular species, yet may be missed if the experiment is not done in the correct animal species. Moreover, many currently available laboratory tests are incapable of detecting certain types of toxic manifestations which occur in man.
Phenotypic complementation, as a way of identifying homologous normal functional genes, is widely used. For example, the human homologue of the yeast cell cycle control gene, cdc 2, was cloned by expressing a human cDNA library in Schizosaccharomyces pombe and selecting those clones which could complement a mutation in the yeast cdc 2 gene (Lee, et al., Nature, 327:31, 1987). A mammalian gene capable of reverting the heat shock sensitivity of the RAS2.sup.val19 gene of yeast, has also been cloned by using complementation (Colicelli, et al., Proc. Nat'l. Acad. Sci. USA, 86:3599, 1989). A rat brain cDNA library was used to clone a mammalian cDNA that can complement the loss of growth control associated with the activated RAS2 gene in yeast. The gene, DPD (dunce-like phosphodiesterase), encodes a high-affinity CAMP phosphodiesterase.
In summary, limitations and uncertainties of existing laboratory tests fail to provide an accurate method of examining the effects of a composition on DNA integrity. In view of this, a considerable need exists for screening methodologies which are inexpensive, rapid, and contain the relevant gene from the animal which is to be treated with the composition. Such methods provide a direct assay to determine if a composition interferes with the DNA repair system of a cell.